Method and apparatus to maintain network coverage when using a transport media to communicate with a remote antenna

ABSTRACT

In a cellular communications system having a centralized radio processing portion (a base station hotel) in communication with a plurality of remote air interface radio portions (or radio heads) over a transport medium, the centralized radio processing portion compensates for a fixed delay associated with the transport medium coupling the centralized radio processing portion and one of the remote air interface radio portions when evaluating a time period corresponding to a variable delay between transmission by a mobile in communication with the one of air interface radio portions and receipt of the transmission by the centralized radio processing portion. The variable delay may relate to time out periods or time slot synchronization.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/661,196, filed Sep. 12, 2003, entitled “Method and Apparatus toMaintain Network Coverage When Using a Transport Media to Communicatewith a Remote Antenna,” naming inventors Ayman Mostafa, Mark Austin, andJohn Carvalho, which claims the benefit under 35 U.S.C. §119(e) ofprovisional application 60/410,136, filed Sep. 12, 2002, whichapplications are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

This invention relates to wireless communication systems and inparticular to timing and delay periods in wireless communicationsystems.

2. Description of the Related Art

Wireless communication systems utilize a timeout period in which thebase station expects to hear from the mobile station. If the basestation does not hear from the mobile station within the timeout period,then the mobile station assumes the transmission has ceased. Inaddition, time division multiple access (TDMA) wireless systems assigntransmission time slots to mobile stations and track the delay of eachmobile station. In such systems, transmission timing of the mobilestations are adjusted, forward or backwards, based on a reception oftransmissions from the mobile station to maintain all the digital timeslots synchronized and prevent adjacent users from having signals thatoverlap each other when they arrive at the base transceiver station(BTS).

For example, if user 2 in timeslot 2 is located directly under the cellsite (where the base transceiver station is located) and user 1 is manymiles away from the base station and is transmitting on timeslot 1, thetime that it takes for the signal to arrive from user 1 at the basetransceiver station is longer than the time it takes for user 2transmissions to arrive, given their different locations with respect tothe cell site. The problem can arise that unless some timing adjustmentis made for when user 1 should transmit, the signal from user 1 intimeslot 1 can start to “overlap” in time with that being transmittedfrom user 2 in timeslot 2. The solution utilized by current systems isfor user 1 in timeslot 1 to “advance” its timing, thereby causing user 1transmissions to arrive earlier and ensuring that they do not interferewith timeslot 2.

For example, in Global System for Mobile Telecommunications (GSM) thedelay is known as Timing Advance (TA), which has allowable values of 0,. . . , 63. One TA unit is equal to one GSM bit (3.7 μs). Theelectromagnetic waves travel at 1.1 Km/bit. That results in a round tripdistance of 1.1×64=70 Km or 35 Km cell radius or a round trip delay of64×3.7 μs=236 μs. Therefore in GSM the maximum cell size is limited to35 kilometers, as this is the maximum timing advance that is allowed.

Referring to FIG. 1, a traditional architecture is illustrated that hasdistributed base transceiver stations 101 around the network 100 withmultiple T1's 103 to each site. A base station controller (BSC) 105controls multiple base transceiver stations 101. While the traditionalarchitecture is functional for current needs, it lacks the ability toshare radio resources efficiently. For example, in one majormetropolitan network, the busy hour traffic carried is about 26 K Erl.,while the network has 60 K voice paths (while being ˜99% digital). Thatsuggests an approximately 26/60=43% efficiency in voice paths (capital).

Consequently, while the mobile switching center (MSC) switch may handlea certain load (simultaneous voice paths) during the busy hour, it isvery common that it takes double the amount of effective voice paths inthe deployed BTSs to handle the same net load on the switch. This is dueto two reasons; 1) the individual cells have slightly different busyhours, and 2) there are no shared resources across BTSs for improvingthe trunking efficiency.

With new spectral efficiency improvements such as advanced multi-rate(AMR) in Global System for Mobile Telecommunications (GSM), and thenewly approaching 3G technologies such as Enhanced Data Rates for GlobalEvolution (EDGE), it is expected that base stations will becomecommonplace that support the traffic equivalent of up to 20 TRXs persector (in greater than 2×10 MHz bandwidths, and assuming full-rate AMR)with speeds of up to 473 kbps per EDGE TRX. Although these types ofcapacities and bandwidths are possible with the network architecture andbuilding blocks of today's cell sites, they seem somewhat inefficientfor meeting the high transmitter/receiver (TRX) demand and high backhaulrequirements of these systems. Indeed, today's base transceiver station(BTS) deployments are entirely distributed where each one is dimensionedindependently for the traffic load it carries in a given busy hour perday.

There are two fundamental areas that operators of mobile communicationsystems attempt to improve on over time, namely capital expenditures(CAPEX), and operating expenditures (OPEX). Under current deploymentstrategies, capital expenditures are expected to grow at a constant ratevs. an incremental minute-of-use (MoU) on the network. Unfortunately,the current base station architectures do not lend themselves towardsimproving OPEX as much as would be desired in a synthesized hoppingnetwork, since radios (with single channel amplifiers) must be combinedusing lossy combiners which results in a coverage loss unless additionalcoax cables are hung on the tower. In addition, every time more radiosare added, the real estate footprint of the cell site itself grows aswell, adding additional rent and lease expense (typical in thosesituations where outdoor shelters are utilized). Additionally, asEnhanced General Packet Switched Radio Service (EGPRS) packet databecomes fully deployed in the network, the number of effective T1 lines(i.e., the required transport bandwidth) between the base stationcontroller (BSC) and the base transceiver station (BTS) is expected toincrease significantly.

Due to these increasing OPEX costs, and no perceived improved CAPEXsavings over time, it would be desirable to provide an alternativearchitecture to help reduce both CAPEX and OPEX for an evolved network.However, that can also have implications for timing advance timeoutperiods. Accordingly, it would be desirable to provide a centralizedarchitecture that can also appropriately deal with timing advance andtimeout issues.

SUMMARY

Accordingly, in one embodiment, the invention resolves limitations ofTime Advance (TA) in a wireless system (e.g. in GSM, TDMA CDMA) when thecell site antenna is remotely located away from processing functionalityand the signal to the antenna undergoes a certain delay, which canreduce the cell radius and limit the radio coverage of the cell and thenetwork. Further, limitations associated with delays between processingfunctionality and the cell site antenna for time out periods are alsoresolved.

In an embodiment, in a cellular communications system having acentralized radio processing portion (a base station hotel) incommunication with a plurality of remote air interface radio portions(or radio heads) over a transport medium, the centralized radioprocessing portion compensates for a fixed delay associated with thetransport medium coupling the centralized radio processing portion andone of the remote air interface radio portions when evaluating a timeperiod corresponding to a variable delay between transmission by amobile in communication with the one of air interface radio portions andreceipt of the transmission by the centralized radio processing portion.The variable delay may relate to time out periods or time slotsynchronization.

In another embodiment a cellular communication system is provided thatincludes a host processing part (a base station hotel) coupled toreceive a communication over a transport medium from a remote airinterface part (a radio head), the host processing part determining atime interval between transmission by a mobile station in communicationwith the remote air interface part (RH) and receipt of the transmissionat the host processing part, the host processing part compensating for afixed delay associated with the transport medium coupling the hostprocessing part and the remote radio interface part in evaluating thetime interval.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a traditional architecture of a mobile communicationsystem.

FIG. 2 illustrates an architecture with fiber based concentratorsaccording to an embodiment of the invention.

FIG. 3 provides the hardware-layered structure architecture of atraditional base transceiver station.

FIG. 4 illustrates functionally an architecture including a BTS hostcoupled to remote radioheads according to an embodiment of theinvention.

FIG. 5 illustrates an example of BTS host with channel/sitemultiplexing.

FIG. 6 illustrates an exemplary embodiment of the basic architecture fora base band unit (BBU) for GSM.

FIG. 7 illustrates the basic architecture of a BBU for use in a WidebandCode Division Multiple Access (WCDMA) system.

FIG. 8 illustrates an exemplary embodiment of multiplexed baseband andsignaling from a BTS hotel to the remote radio head using time divisionmultiplexing for the various sites.

FIG. 9 illustrates an exemplary transmitter configuration for aradiohead.

FIG. 10 illustrates an exemplary embodiment of the receive portion of aradio head.

FIG. 11 illustrates an exemplary embodiment of the BTS host accountingfor the fixed fiber delay via increases to the timeout counter.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The centralized architecture shown in FIG. 2 allows sharing of radioresources for cell sites from a central location. Such an architectureprovides the potential for significant hardware savings sincepooled-radio/channel processing units can be shared across multiple cellsites. The centralized architecture can provide centralized management,which can provide savings in operation and maintenance as well aslending itself to utilization of software defined radio concepts. Thearchitecture illustrated in FIG. 2 utilizes centralized base stationhotels 201 and 202 (also referred to herein as hosts), and replacestoday's base transceiver stations (BTSs) by much simpler radio heads(RHs). In one embodiment, the RHs provide little more thandigital/analog (D/A) conversion and radio/frequency (RF) amplificationand filtering.

One goal of the architecture described herein is to provide a ubiquitousand affordable access to each RH by its BTS host. Referring again toFIG. 2, according to an embodiment, the BTSs are concentrated into a fewlocations, e.g. BTS 201 and 202 and are coupled with remote “dumb” radioheads 203, 205, and 207 at the required RF emitting locations.Site-specific processing units are kept at the BTS hotel point.Preferably, there are only a few BTS hotels in a typical metropolitanarea and they should preferably exist at the POP points (points ofpresence) or at a Mobile Telephone Switching Office (MTSO) location tominimize the cost of cross-connect between BSC/MSC and BTS host. Fromthe transport point of view, the BTS hotel concentrates the transportsignal of the individual sites to aggregate the traffic to the BSC. Forthat reason the BTS hotel could preferably be located with or close tothe BSC.

The distribution between the BTS host and RHs can be any high bandwidthmedium. For example, as shown in FIG. 2, connection between BTS hotel202 and RH 205 could be implemented through a line of site wireless linkutilizing free space optics (FSO). Also, connection between BTS hotel202 and RH 207 could be implemented through microwave (MW)communications. However, in many cases, no line of site exists, so thatalternative backhauls such as fiber optics can be utilized. Use ofoptical fiber as the transport medium has several technical advantages.For example, optical fiber does not exhibit fading or multiple pathimpairments. Further, its performance is not a function of the weatheror geographical changes. Optical fiber distribution can also beparticularly attractive due to its bandwidth potential, and relativeabundance in metro areas. While many of the examples provided hereinutilize optical fiber, note that the concepts described herein are alsoapplicable to other high bandwidth mediums. In one embodiment, thedigital transport is point to multipoint. Note that the BTS host and RHarchitecture may be based on Software Design Radio concepts, whichprovides the capability of supporting multiple communication standardson the same platform.

Although examples provided herein explaining transmitting RF over fiberin accordance with embodiments of the invention are described primarilyfor GSM, they are equally relevant for other technologies such as timedivision multiple access (TDMA), code division multiple access (CDMA),wide band CDMA (WCDMA) and orthogonal frequency division multiplexing(OFDM) across any bands.

There are several BTS Hotel/RH transmission options for RF over fiber.Implementing a transmission option entails determining an appropriatedividing point for the traditional BTS design into a BTS host and RHwhere the tradeoff is between backhaul bandwidth requirements andcomplexity (and expense) of the RH. FIG. 3 provides, from a hardwareperspective, the hardware-layered structure architecture of atraditional base station to provide a framework for the discussion ofthe various possible dividing points to break the traditional BTS designinto a BTS hotel and the RH. One part of the hardware structure is theAbis interface 301 that provides the communication interface with theBSC/MSC. A second part of the hardware structure 303 provides baseband/DSP processing. A third part of the hardware structure 305 providesintermediate frequency (IF) processing and the fourth part of thehardware structure provides the radio part 307 for RF transmission.

Several options are provided below for an appropriate dividing pointbetween the BTS hotel and the radio head. Assume (for an embodiment)that the RF, IF, or base band data signal is transformed into an opticalsignal and transmitted over a fiber optic link. On the remote end, theoptical signal is converted back to RF, IF, or a data signal and thenprocessed, amplified and sent to the antenna. The following sectiongives an overview of the bandwidth requirement for each case.

One option is to transmit RF signals over fiber. That transmissionoption is considered analog over fiber. The RF carriers modulate theoptical signal through the fiber. Since the modulation is based on laserintensity, no other digital data signal can be modulated and used alongwith the RF in the fiber. Note that electric to optic (E/O) and optic toelectric (O/E) conversion loss in addition to the long haul fiber losscould result in significant RF loss that would reduce the spurious freedynamic range (SFDR) of the receiver. Thus, transmission of this sortmay be regarded as distance-limited.

Another option is for transmission of intermediate frequency signalsover the fiber. In this case the bandwidth (BW) required can beestimated as follows. Assume a 12.5 MHz over IF signal for the downlink,with 52 mega samples per second (MSPS). Using commercially availableanalog/digital (A/D) converters the required bandwidth is 52 MSPS*14bits/samples ˜730 Mbps for the downlink. Assuming receiver diversity forthe uplink, twice that bandwidth or 2*730 Mbps is needed. The total BWper site would be 730*3˜2190 Mbps plus overhead site specifics,parameters, alarms and the like. Thus, transmission of intermediatefrequency signals over fiber utilizes OC-48 BW capability (2.4 Gbits/s)per site.

Another option is for baseband transmission of RF over fiber. With thisapproach, only the traffic data to the specific site is carried over thefiber. In one embodiment, the composite signal to every site is built atthe BTS host and transported to the RH over the fiber link. Theestimated BW per carrier is dependent on the technology. For GSM, theair interface BW is about 270 Kbps, while for WCDMA it is 3.84 Mbps.Higher transport bandwidth requirements may be required according towhether synchronization is maintained in the radio head or in the BTShotel. The baseband option results in the lowest backhaul requirements,as a tradeoff for slightly higher complexity in the RHs since each radiohead now provides IF processing capability. The BTS host and RHarchitecture themselves for this option are described in more detail inthe following sections.

FIG. 4 illustrates functionally an embodiment of a BTS host 401 (hotel)for a baseband transmission strategy. The sites (Site 1. Site n) 403represent the baseband radio resources needed for a given cell site at aparticular time. Each site corresponds to an RH supported by the BTShost. The baseband units (BBU) 407 process the traffic/control signalsfor its site (site 1 or site n). There are two levels of multiplexing inthe illustrated embodiment. The first level of multiplexing isrepresented by multiplexers 409. The multiplexers 409 are on the sitelevel and multiplex all channels within one site. Thus, the basebandprocessing occurring for all the channels of the site are selectivelymultiplexed by multiplexers 409. A second level of multiplexing isrepresented by multiplexer 411 and occurs at the BTS host level.Multiplexer 411 functions to multiplex baseband signals for the multiplesites supported by host 401 for transmission over optical ring 413.

As illustrated in FIG. 4, the BTS host 401 provides a central locationfor pooling the radio processing units of all channels in the network.Preferably, there are one or two BTS hosts in the whole network. In alarge network, such as those of a big metropolitan area, a few BTS hostsare practical. The numbers and locations of BTS hosts depend on thenumber of the RHs and their locations. BTS hosts can also be used toprovide redundancy to the network in case of transport failure or poweroutage. However, such redundancy typically requires extra transportcost. Note that in the embodiment illustrated in FIG. 4, some or all ofthe baseband processing capability may be dedicated to a particularsite.

In an embodiment implementing baseband transmission over fiber, theIF/RF sections of the BTS radio are located remote from the host andclose to the tower in the RH. Each RH could have one or a multiplenumber of sectors.

In one embodiment wavelength division multiplexing (WDM) is used tocombine one or more wavelengths over fiber 413. Different wavelengthmultiplexing schemes can be used depending on the transported data rate:For example, one wave length division multiplexing scheme is known asCoarse Wave Division Multiplexing (CWDM). Another wave length divisionmultiplexing scheme is known as Dense Wave Division Multiplexing (DWDM).CWDM combines a relatively low number of wavelengths (e.g., 6), whileDWDM combines 32 wavelengths in protected mode (i.e., for everywavelength, a back-up wavelength is also transmitted for failureprotection) and 64 in non-protected mode. Referring still to FIG. 4,Optical Add and Drop Multiplexers (OADM) 415 are used to multiplex theoptical wavelengths in and out RH around the ring 413. Each RH may beassigned one or more wavelengths and different wavelengths may be usedfor the uplink and downlink. On the BTS host side, wavelength divisionmultiplexers (WDM) 417 and 419 multiplex/demultiplex multiple opticalwavelengths in the downlink/uplink (DL/UL) to meet the network data rateneeds. For example, baseband RF data supplied from each site's basebandunits are supplied by WDM 417 at a different wavelength to optical ring13.

Referring to FIG. 5, effective radio resource allocation can beaccomplished by using a switching matrix 501 that routes any radioresource (BBU) within the BTS Host 500 to any remote RH. The channel andsite-switching matrix 501 provides the dynamic radio allocation feature.That allows baseband processing resources to be allocated to a siteaccording to its varying utilization of resources. The switching matrixcan be looked at as a time switch for all BBU units. Thus, any BBU canbe allocated to any particular site. The site multiplexer 503 followingthe channel/site switching matrix 501 can be incorporated inside theswitch matrix. The data compression (DC) modules 503 and 507 areoptional data compression modules that compress the data transportedover the fiber. In a preferred embodiment, the BTS host is configured toallow the base band resources to be dynamically allocated to any cellsite within the network as needed. In certain embodiments, all thebaseband resources can be dynamically allocated, while in otherembodiments, only a portion of the baseband resources are dynamicallyallocated to the various sites.

Referring to FIG. 6 the basic architecture of a BBU for GSM (singlecarrier) is illustrated. One potential division point between BBU and RHblocks is indicated by arrow 601, which shows that the data rate at theindicated break point is 271 kps. The blocks shown in FIG. 6 arestandard processing blocks and are known to those of skill in the art.As shown in FIG. 6, voice data is received from the BSC at block 603.Error coding is performed in block 605 with interleaving anddifferential encoding performed in blocks 607 and 609. Integration and aGaussian low pass filter are implemented in blocks 611 and 613. In oneimplementation, the signal at that point is forwarded over the opticalring to the RH for serial to parallel (S/P) conversion at 610 andadditional processing and RF transmission. Note that in otherembodiments, the baseband unit may include additional processingfunctions. For example, in one embodiment, all the processing up to thein phase and quadrature (I/Q) modulation unit 615 is performed in theBBU.

FIG. 7 illustrates the basic architecture of a BBU for use in a WidebandCode Division Multiple Access (WCDMA) system. As shown in FIG. 7, voicedata is received from the BSC at block 703. Error coding is performed inblock 705 with interleaving performed in block 707. Block 708 providesan orthogonal variable spreading factor (OVSF), which is added to thesignal in block 709. In one implementation, the signal at that point(701) is forwarded over the optical ring to the RH for serial toparallel (S/P) conversion at 710 and additional processing includingcomplex spreading in block 711, low pass filtering operations in block715, I/Q modulation in block 717, and RF transmission. Note that theprocessing blocks illustrated in FIGS. 6 and 7 are known to those ofskill in the art and will not be further described herein. Note thatother division points between the processing of the BBU and the RH maybe utilized by other embodiments of the invention.

An alternative to the implementations illustrated in FIGS. 6 and 7 isfor the BBU to construct the composite signal for a given site by theBBU. The composite signal is then digitized and is transmitted to the RHfor IF and RF frequency conversion. The tradeoff of this approach isadded complexity in the baseband unit and reduced complexity in the RH,while potentially increasing the bandwidth requirements from the host tothe RH.

In one embodiment, the channels and sites in the downlink (DL) anduplink (UL) are time multiplexed to reduce the transport requirements.In one embodiment, the baseband air interface data are transmitted overthe fiber and the transmission over the fiber is a digital transmission.An example of multiplexing in the downlink is shown in FIG. 8. Thebandwidth needed in this case for the downlink=B*RH, where B isbits/second per carrier and RH is the number of radio heads. It is alsoassumed that the application used is based on one time slot per mobile.Half Rate AMR users could assume twice the number of users forcomparable bandwidth utilization as full rate. On baseband level: Thebandwidth needed per carrier is 270 kHz. Assuming sampling at 2×(Nyquist) the sampling frequency is 540 kHz. With 14 bit resolution, thetotal BW needed is =7.6 Mbps. For 20 carriers per sector, and 3 sectorsper site, the required transmission is approximately 450 Mbps. Withreceive diversity, the required bandwidth for the uplink isapproximately twice the downlink requirements of 900 Mbps. That iscomparable to the bandwidth needed to digitize 12.5 MHz spectrum at theIF. Note that having fewer carriers results in a reduced bandwidthtransmission. For example, with a site having four carriers per sectorand three sectors per site will require about 180 Mbps transmissionbandwidth (assuming diversity). Due to the current technology limitationof commercial A/D, it is not feasible to do IF digitization of the wholeIF spectrum for PCS band (60 MHz). Note that in-band signaling andsupervisory alarms/commands between BTS host and RH do not constitute asignificant overhead.

Using an optical network of OC-192 (˜9.6 Gbps) for the downlink impliesthat the number of radio heads that can be supported is 20 radio heads.Twice as much bandwidth needs to be allocated to support 20 radio headsfor the uplink (assuming diversity). The number of mobiles served byeach site, assuming full utilization (1/1 reuse, FH, full-rate AMR and40% frequency loading) is about 190 mobiles. The total number ofsimultaneous users supported by one BTS host is therefore about 3800 forthe downlink. The bandwidth for each uplink diversity path is equivalentto the downlink since both are using channel multiplexing. However,diversity requires approximately double the transmission bandwidth.

FIG. 8 illustrates an exemplary embodiment of multiplexed baseband andsignaling from a BTS hotel to the remote radio head using time divisionmultiplexing for the various sites. As shown, each site, e.g., site 1requires information packets for various channels 1, 2, and 62. Amultiplexer or switch 801 selects which information packet is to beselected for site 1 at any particular time. Similarly a multiplexer orswitch 803 selects which information is to be provided to site n. Amultiplexer 805 is used to combine the packets or information units tobe supplied to the fiber network. The fiber network then receives thevarious packets destined for the various RH sites, which demultiplex thepackets according to signaling (control) information supplied. In-bandcontrol messages and supervisory signals between the BTS hotel and theRH are carried over the fiber connection to define RH parameters, sitechannels, alarms and the like. Critical commands and alarms between RHand BTS are needed for the proper operation of the architecturedescribed herein. For example, such information as output power, antennavoltage standing wave ratio (VSWR) monitoring, tower mounted amplifier(TMA) detection, site environment, TX and RX antenna detection, needs tobe communicated between the RH and the BTS hotel. Such control overheadgenerally does not add significantly to bandwidth requirements. Anaddressing scheme may be used, in addition to or instead of WDM, toallow the RH to extract its dynamically allocated bandwidth out of theTDM highway between BTS host and RH over the fiber ring. RH remoteaddressing should be sufficiently robust to allow distinguishing thenumber of RHs supported by the BTS host. Multi-cast and broadcastaddressing schemes may also be desirable. Error detection protocols,e.g., Link Access Protocol D (LAPD), may be utilized for communicationbetween RH and BTS hotel to enable the processing units on both sides totolerate errors (and therefore not have to throw away all frames witherrors). In addition the bit error rate (BER) may be monitored on thelink.

One goal for partitioning the BTS host and RH hardware may be tominimize the transport lease of the fiber ring. Multiplexing of theuplink and downlink allows efficient BW utilization over the network,and therefore cost saving over the transport media.

In one embodiment, the remote RH is a simplified BTS with reducedcomplexity and cost. For example, the baseband processing has been movedinto the BTS hotel. The RH covers the same cell area that is covered bycurrent BTSs and can utilize existing poles, antenna, buildings andother structure. With such reduced functionalities and size the sitespace needed will be reduced resulting in lower lease costs over time. Asimplified block diagram of an exemplary RH is shown in FIGS. 9 and 10.FIG. 9 shows the transmit part of the radio head and FIG. 10 shows thereceive part of the radio head.

An exemplary transmit configuration 900 is shown in FIG. 9 for onesector. The demultiplexer 901 demultiplexes the optical signal from theSONET ring for the particular site and provides digital data to thetransmit programmable filter bank (TX PFB) 903, which functions as aprogrammable waveform generator and filter bank, building the compositesignal provided to digital to analog converter (DAC) 905. Theprogrammable waveform generator and filter bank provide an appropriatefrequency shift for each channel being transmitted to build thecomposite signal and is shown in more detail in block 911 wheree^((jFt1) ^(—) ^(s1)) represents the transmit frequency 1 (Ft1) for site1 (s1). The various functional blocks illustrated in FIG. 9 are known inthe art and therefore will not be described in detail. The compositeradio signal is constructed using the filter bank, translated to RF(first converted to the intermediate frequency (IF) and then the radiofrequency (RF) using the local oscillators (lo) in a manner known in theart) and amplified through a Multicarrier Power Amplifier (MCPA). Notethat in some embodiments, the composite signal generated in FIG. 9 outof block 903 is generated in the base station host and transmitted overthe transport medium.

Another alternative (not shown in FIG. 9) is to run individual paralleltransmit channels in the RH for every carrier with its own SingleCarrier Power Amplifier (SCPA) and provide a power combiner at theantenna point. Low power radios coupled with Multi-carrier poweramplifiers or channelized Single Power Carrier Amplifier (SCPA) withcombiner systems may be used to amplify the carriers at the antenna end.One embodiment may utilize very simple low cost RHs where there are fewTRXs, e.g., one or two TRXs, and SCPAs can be used in theseapplications.

FIG. 10 illustrates an exemplary embodiment of the receive portion 1000of the RH. The antenna receives the transmitted signal and provides thereceived signal to band pass filter (BPF) and low noise amplifier (LNA).The processing of the received RF signal is conventional. The receivercarriers are separated through receive filter banks 1001 (additionaldetails of which are shown at block 1003) and are multiplexed, usingtime division multiplexing (TDM), for transport over the fiber asdescribed previously. Uplink multiplexing is similar to the downlinkmultiplexing except two receive antennas are used per site. Therefore,the bandwidth used with diversity is twice that carried over the fiberring if no diversity was used. In addition to allowing the transmissionand reception of the carriers of interest on a given sector, it is alsodesirable for the RH to have the ability to allow for a scanningreceiver for a variety of functions such as; uplink interferencemonitoring, frequency time-slot measurements of other mobile stations(providing, e.g., for the least interfered traffic channel, based onuplink measurement done by the RH and mobile measurement report, to bechosen for the call). Interference monitoring should be continually doneby the RH and reported back to the BTS host. The scanning receiver isconsidered part of the interference measurements for Dynamic ChannelAllocation (DCA). Note that the embodiment illustrated in FIG. 10 isexemplary and more or less processing may be performed in the radiohead.

As described above, wireless communication systems utilize a timeoutperiod during which the base station expects to hear from the mobilestation. In addition, delay is important in appropriately adjustingtiming advance associated with the mobile stations. In the architecturedescribed above utilizing optical fiber for the transport medium, adelay is incurred in transporting signals between the base station hoteland the radio head. The signals travel through the optical fiber at 0.68the speed of free space and the delay encountered through the fiber hasto be accounted for by the BTS hotel. As described above, in GSM, the TA(time advance) allowable delay is 63 bit units for round trip delay or233 μsec. That is equivalent to a cell radius of 35 Km (waves travel 1.1Km in free space every GSM air interface bit −3.7 μsec-). The mobile isnot allowed to transmit if it is more than 35 Km away from the BTS sincethe burst transmission would be received in the next time slot assignedto another connection even if the timing advance was maximized. Unlessotherwise accounted for, the 35 Km cell radius has to be reduced by theequivalent delay through the fiber. The equivalent cell radius if theelectromagnetic wave travels through fiber is 35×0.68=24 km. Therefore,if the length of the fiber is 24 km, the allowable cell radius remainingfor the wireless portion is zero. That would severely limit the opticaltransport system proposed herein as the desire is to serve many remoteradio heads at significant distances from a base station hotel.

One solution is to calibrate the delay introduced by transmissionthrough the optical fiber between the BTS hotel and each optical spliceor remote RH at the time of the installation and periodically thereafterand the BTS hotel can account for the delay in determining time outand/or timing advance periods. Note that since the fiber delay is fixed,it does not cause any more potential overlap or adjacent time slotcross-talk problems than that already introduced by the variable delayfrom the wireless transmission. In one embodiment, the fixed delay isbacked out of any timing calculations performed by the BTS hotel. Thatis, when a timing parameter is received indicating when a communicationwas sent by the mobile, the calculated time period between being sent bythe mobile and received by the BTS hotel is reduced by the fixed delayassociated with the transport medium for communicating with theparticular radio head in communication with the mobile requiring timingadvance or timeout evaluation. Each of the radio heads is going to haveits own fixed delay period based on its fiber delay to the BTS hotel andthe BTS hotel utilizes the appropriate fixed delay according to which ofthe radio heads it is communicating with.

In one embodiment, the BTS host takes this extra fixed fiber delay intoaccount via increases, which may be implemented in software, to thetimeout counter as shown in FIG. 11. The mobile and the BSC both processthe signal normally as if no extra delay were introduced by the fibertransmission. The new timeout is defined as the Virtual Timing Advance(VTA) for a particular mobile. All RF radio frequencies or carriersconnected to that site, and hence their respective mobiles, incur thesame delay caused by the optical fiber. The BTS software assumes thatconstant delay during the synchronization to all mobiles of thatparticular site. Referring still to FIG. 11, the BTS software requeststhat the respective mobile advances its transmission by the normal delay(TA n), which is the virtual delay minus fiber delay (VTA n-FD). The BTSstill reports to the BSC only the “TA n” part of the delay for use byany algorithms utilizing the TA implemented by the BSC. In prior artsystems, the base transceiver station (BTS) was located in closeproximity to the air interface with the mobile station so that the delaydue to communication with the BTS could be neglected.

A transmission by the mobile includes a time stamp. The BTS hostreceives that time stamp and determines a delay associated withreceiving that transmission from the mobile. That delay is the VTA andincludes both the FD and TA n (FIG. 11). The host can either add thefixed delay to its TA n calculations when comparing to the VTA orsubtract out the fixed delay from the VTA.

In one embodiment, the BTS uses a virtual TA (VTA) of up to 63+156 bits(one time slot extra delay) and reports the TA delay to the BSC insteadof VTA, since the extra delay incurred by the fiber can be accounted forconceptually. Larger delays than 156 bits might not be practical sincethe amount of jitter and wander inherent in larger delays couldcomplicate radio synchronization. The fiber delay between the BTS hostand RH can be calibrated at installation time and periodically throughthe network. Various methods are known to determine delay of thetransport medium. For example, a packet or frame may be transmitted bythe BTS hotel that is returned to the BTS hotel. The BTS hotel candetermine the round trip time to determine the delay inherent in thetransport medium. Alternatively, a global positioning system (GPS) timestamp may be inserted in a packet or frame sent between the BTS hoteland the RH with the receiving side utilizing a GPS time stamp on receiptto enable determination of delay caused by the fiber (or other transportmedia).

One goal of any architecture is to decrease the CAPEX and OPEX costs ofthe system over time. Software defined radio (SDR) provides theversatility to support multiple standards on the same platform. SDR maybe utilized to provide versatility in the evolution to WCDMA, High SpeedDownlink Packet Access (HSDPA), as well as supporting other airinterfaces such as 802.XX. To this end, one BTS host architecture isbased on standard off-the-shelf platforms that run standard operatingsystems, where the air interfaces are pure software implementations ontop of the stand platforms. For instance, the evolution of PCarchitectures with standard off the shelf microprocessors in redundantserver architecture and shared operating systems such as Linux arepossible implementation solutions for the BTS host. In this embodiment,an operator buys the software libraries for a certain release from a GSMvendor, which would run on a standardized platform. In this manner,upgrades and the cost of the BTS host platform will naturally follow theefficiency and cost curves of the PC industry. Reliability is of coursean important criterion for hardware. Note that application of SDRconcepts to the radio head may be less advantageous as most of thefunctionality in the radio head is processing or amplification that isneeded anyway even if all the extra processing was done in software.

A variety of dynamic resource sharing algorithms and techniques methodsand concepts are possible with the BTS host, RH architecture describedherein. In addition to being able to dimension radio resources onequivalent to a switch or BSC level, rather than needing to dimensioneach BTS to satisfy the demands of its individual busy hour, there areother radio resource efficiencies and algorithms that are possible withthis architecture. For instance, simulcast capability is provided bysending the signal to two RHs simultaneously. Bandwidth efficientimplementations (over the fiber ring) of simulcast may have both radioheads listen to the same signal being sent over fiber as well. That is,a multicast capability is provided over the fiber ring so that multipleradio heads can be targeted for a particular communication from the BTShotel. Macro-diversity can be also achieved by having two radio headslisten to the same signal, which is then combined in the BTS host. SmartAntenna beam steering can also be conceptually done by sending thesteering signals to multiple RHs which are estimated to provide animproved signal to a given mobile, or nulling towards an interferer. Inaddition, smart resource allocation for hopping signals in asynchronized network to avoid collisions can also be implemented.

In order to minimize the cost associated with the proposed architecture,the current BTS architecture should be configured to allow anappropriate interface to the network model described herein. Inaddition, it is desirable to standardize the interface of the BBU and RHbetween multiple vendors so that different vendors can share thenetwork.

In GSM and EDGE, the power control is not as timely and critical as inCDMA2000 or WCDMA. The RH needs to periodically report the output powerand antenna monitoring signals at the remote side to the BTS host. If RFsignals are transferred over the fiber (rather than baseband or IF), theRH needs to account for the loss through the fiber ring to maintain theproper output power level at the antenna and prevent saturating thepower amplifier (PA). The BBU can provide the proper power control levelon a time slot basis.

In one embodiment, the down link power control for a given user (slot ina TDMA system) is done in the BBU. The RH should maintain a constantgain in the up and down links for all the channels. A power monitoringfunction should exist in the RH for the downlink and looped back to theBTS host respective site supporting the RH for further powerequalization and balance.

The radio heads should be synchronized to the same BTS host. Onesynchronization approach is to use the GPS currently used in positioningto time stamp a GSM frame, or alternatively a new sync frame or worddefined for the fiber transport. BTS host to BTS host synchronizationmay be accomplished by utilizing synchronization messages between thehosts that compare the time stamps of their respective GPS receiver.

Centralized operation and maintenance are one of the advantages that thevarious embodiments of the invention described herein provide over theexisting scheme.

As in any architecture change, the new architecture needs to be phasedin with the existing architecture. New equivalent base stations could bedeployed with the new architecture; however, it may also be beneficialto pull back the backhaul of the existing systems in some multiplexingfashion on the fiber ring as well. One embodiment of a network based onthe new architecture is to use a large number of radio head and lowertransmitter power. The RHs could also be deployed in the existing sitesto maintain a uniform low output distribution, which allows the mobileto operate at a lower output power thereby extending the batterylifetime.

While various aspects of a new architecture have been described herein,note that the description of the invention set forth herein isillustrative, and is not intended to limit the scope of the invention asset forth in the following claims. For example, while a network ring hasbeen described utilizing SONET, various networks, technologies, ortransport media such as PDH, ATM, or Ethernet networks could all be usedto transport signals between the BTS hotel and the RH. The link betweenthe BTS hotel and the RH may be a point to point connection implementedin FSO or MW. Also, various processors may be used and variousimplementations of the link are possible. Other variations andmodifications of the embodiments disclosed herein, may be made based onthe description set forth herein, without departing from the scope ofthe invention as set forth in the following claims

1. A method for use in a cellular communications system having acentralized radio processing portion in communication with a pluralityof remote air interface radio portions over a transport medium, themethod comprising: in the centralized radio processing portion,compensating for a fixed delay associated with the transport mediumcoupling the centralized radio processing portion and one of the remoteair interface radio portions in evaluating a time period correspondingto a variable delay between transmission by a mobile station and receiptof the transmission by the centralized radio processing portion, themobile station communicating with the one of air interface radioportions, the communication being received by the centralized radioprocessing portion from the one of the remote air interface radioportions; and providing to the mobile station a timing adjust valueindependent of the fixed delay; wherein the timing adjustment value isused to avoid overlap in time slots with another mobile station.
 2. Amethod for use in a cellular communications system having a centralizedradio processing portion in communication with a plurality of remote airinterface radio portions over a transport medium, the method comprising:in the centralized radio processing portion, compensating for a fixeddelay associated with the transport medium coupling the centralizedradio processing portion and one of the remote air interface radioportions in evaluating a time period corresponding to a variable delaybetween transmission by a mobile station and receipt of the transmissionby the centralized radio processing portion, the mobile stationcommunicating with the one of air interface radio portions, thecommunication being received by the centralized radio processing portionfrom the one of the remote air interface radio portions; and wherein theevaluating is determining a time out period associated with thetransmission from the mobile station to determine if the transmissionwas received within the time out period.
 3. The method as recited inclaim 1 wherein the evaluating is for synchronizing calls by adjusting atransmission timing of the mobile station according to the evaluation ofthe time period corresponding to the variable delay.
 4. The method asrecited in claim 1 further comprising utilizing a calibrated value forthe fixed delay.
 5. The method as recited in claim 1 wherein a pluralityof remote air interface radio portions are coupled via the transportmedium to the centralized radio processing portion, and wherein thecentralized radio processing portion accounts for a different fixeddelay for each of the remote air interface radio portions.
 6. The methodas recited in claim 1 wherein the transport medium is optical fiber. 7.The method as recited in claim 1 wherein the transport medium is one offree state optical and microwave.
 8. The method as recited in claim 1further comprising supplying a base station controller (BSC) with thevariable delay.
 9. A cellular communication system comprising: a hostprocessing part coupled to receive a communication over a transportmedium from a remote air interface part, the host processing partdetermining a time interval between transmission by a mobile station incommunication with the remote air interface part and receipt of thetransmission at the host processing part, the host processing partcompensating for a fixed delay associated with the transport mediumcoupling the host processing part and the remote radio interface part inevaluating the time interval; and wherein the evaluating determines ifthe transmission from the mobile station was received within anallowable timeout period.
 10. The cellular communication system asrecited in claim 9 comprising a counter coupled to account for the fixeddelay.
 11. The cellular communication system as recited in claim 9wherein the counter is implemented in software.
 12. The cellularcommunication system as recited in claim 9 wherein the time interval isa sum of a first time period corresponding to receipt of thetransmission at the remote air interface part and the fixed delay. 13.The cellular communication system as recited in claim 9 wherein thetimeout period is evaluated by adding the fixed delay to the allowabletime out period and comparing to the time interval.
 14. The cellularcommunication system as recited in claim 9 wherein the timeout period isevaluated by subtracting the fixed delay from the time interval andcomparing to the allowable time out period.
 15. A cellular communicationsystem comprising: a host processing part coupled to receive acommunication over a transport medium from a remote air interface part,the host processing part determining a time interval betweentransmission by a mobile station in communication with the remote airinterface part and receipt of the transmission at the host processingpart, the host processing part compensating for a fixed delay associatedwith the transport medium coupling the host processing part and theremote radio interface part in evaluating the time interval; and whereinthe time interval corresponds to a timing advance time period summedwith the fixed delay, the timing advance period being used to avoidoverlap in time slots with another mobile station.
 16. The cellularcommunication system as recited in claim 9 wherein the fixed delay is ameasured value.
 17. The cellular communication system as recited inclaim 9 further comprising a plurality of remote radio interface partscoupled via the transport medium to the host processing part, andwherein a different fixed delay is associated with each of the remoteair interface parts.
 18. The cellular communication system as recited inclaim 9 wherein the transport medium is an optical fiber.
 19. Thecellular communication system as recited in claim 9 wherein thetransport medium is one of free state optical and microwave.
 20. Thecellular communication system as recited in claim 9 wherein the hostprocessing part is further coupled to receive respective communicationsover the transport medium from a plurality of other remote air interfaceparts, each of the other remote air interface parts having respectivefixed delays over the transport medium different from the fixed delayassociated with the remote air interface part.
 21. A cellularcommunication system comprising: a host processing part coupled toreceive a communication from a mobile station via a transport medium;and means for compensating for a fixed delay associated with a transportmedium coupling the host processing part and a remote radio interfacepart when evaluating a timing period associated with a transmission fromthe mobile station in communication with the remote radio interfacepart; and wherein the timing period is one of a time out periodassociated with dropping a call if the transmission is not receivedwithin the time out period and an adjustable timing advance provided tothe mobile station independent of the fixed delay that is used to avoidoverlap in time slots with another mobile station.